Control rod/control rod drive mechanism couplings

ABSTRACT

A nuclear reactor includes a pressure vessel, and a control rod assembly including at least one movable control rod comprising a neutron absorbing material, a control rod drive mechanism (CRDM) for controlling movement of the at least one control rod, and a coupling operatively connecting the at least one control rod and the CRDM. The coupling includes a first portion comprising a first material having a first density at room temperature, and a second portion comprising a second material having a second density at room temperature that is greater than the first density. In some embodiments the coupling includes a connecting rod including a hollow or partially hollow connecting rod tube comprising a first material having a first density and a filler disposed in the hollow or partially hollow connecting rod tube, the filler comprising a second material having a second density greater than the first density.

BACKGROUND

The following relates to the nuclear power generation arts, nuclearreaction control arts, control rod operation arts, and related arts.

In known nuclear power plants, a nuclear reactor core comprises afissile material having size and composition selected to support adesired nuclear fission chain reaction. To moderate the reaction, aneutron absorbing medium may be provided, such as light water (H₂O) inthe case of light water reactors, or heavy water (D₂O) in the case ofheavy water reactors. It is further known to control or stop thereaction by inserting “control rods” comprising a neutron-absorbingmaterial into aligned passages within the reactor core. When inserted,the control rods absorb neutrons so as to slow or stop the chainreaction.

The control rods are operated by control rod drive mechanisms (CRDMs).In so-called “gray” control rods, the insertion of the control rods iscontinuously adjustable so as to provide continuously adjustablereaction rate control. In so-called “shutdown” control rods, theinsertion is either fully in or fully out. During normal operation theshutdown rods are fully retracted from the reactor core; during a SCRAM,the shutdown rods are rapidly fully inserted so as to rapidly stop thechain reaction. Control rods can also be designed to perform both grayrod and shutdown rod functions. In some such dual function control rods,the control rod is configured to be detachable from the CRDM in theevent of a SCRAM, such that the detached control rod falls into thereactor core under the influence of gravity. In some systems, such asnaval systems, a hydraulic pressure or other positive force (other thangravity) is also provided to drive the detached control rod into thecore.

To complete the control system, a control rod/CRDM coupling is provided.A known coupling includes a connecting rod having a lower end at whichthe control rod is secured. The upper portion of the connecting rodoperatively connects with the CRDM. A known CRDM providing gray rodfunctionality comprises a motor driving a lead screw that is integralwith or rigidly connected with the connecting rod, such that operationof the motor can drive the lead screw and the integral or rigidlyconnected connecting rod up or down in a continuous fashion. A knownCRDM providing shutdown functionality is configured to actively hold thecontrol rod in the lifted position (that is, lifted out of the reactorcore); in a SCRAM, the active lifting force is removed and the controlrod and the integral or connected connecting rod fall together towardthe reactor core (with the control rod actually entering into thereactor core). A known CRDM providing dual gray/shutdown functionalityincludes a motor/lead screw arrangement, and the connection between themotor and the lead screw is designed to release the lead screw duringSCRAM. For example, the motor may be connected with the lead screw via aseparable ball nut that is actively clamped to the lead screw duringnormal (gray) operation, and separates in the event of a SCRAM so thatthe control rod, the connecting rod, and the lead screw SCRAM together(that is, fall together toward the reactor core).

Related application Ser. No. 12/722,662 titled “Control Rod DriveMechanism For Nuclear Reactor” filed Mar. 12, 2010 and relatedapplication Ser. No. 12/722,696 titled “Control Rod Drive Mechanism ForNuclear Reactor” filed Mar. 12, 2010 are both incorporated herein byreference in their entireties. These applications discloseconfigurations in which the connection between the motor and the leadscrew is not releasable, but rather a separate latch is provided betweenthe lead screw and the connecting rod in order to effectuate SCRAM. Inthese alternative configurations the lead screw does not SCRAM, butrather only the unlatched connecting rod and control rod SCRAM togethertoward the reactor core while the lead screw remains engaged with themotor.

The CRDM is a complex device, and is typically driven electricallyand/or hydraulically. In the case of shutdown or dual gray/shutdownrods, the control rod system including the CRDM may also be classifiedas a safety related component—this status imposes strict reliabilityrequirements on at least the shutdown functionality of the CRDM.

To reduce cost and overall system complexity, it is known to couple asingle CRDM with a plurality of control rods via an additional couplingelement known as a “spider”. In such a case all the control rods coupledwith a single CRDM unit move together. In practice a number of CRDMunits are provided, each of which is coupled with a plurality of controlrods, so as to provide some redundancy. The spider extends laterallyaway from the lower end of the connecting rod to provide a large“surface area” for attachment of multiple control rods. The spidertypically comprises metal tubes or arms extending outward from a centralattachment point at which the spider attaches with the connecting rod.In some spiders, additional supporting cross-members may be providedbetween the radially extending tubes. The diameters (or more generally,sizes) of the metal tubes or arms comprising the spider are kept as lowas practicable in order to minimize hydraulic resistance of the spiderduring SCRAM and to enable the control rod support structure to contactand cam against all control rods during raising or lowering of thecontrol rods.

The coupling comprising the connecting rod and the spider is arelatively lightweight structure that minimizes material cost andweight-loading on the complex CRDM. For various reasons such as strengthand robustness, low cost, manufacturability, and compatibility with thereactor vessel environment, both the connecting rod and the spider areusually stainless steel elements.

BRIEF SUMMARY

In one aspect of the disclosure, an apparatus comprises: at least onecontrol rod comprising a neutron absorbing material; a control rod drivemechanism (CRDM) unit; and a control rod/CRDM coupling connecting thecontrol rod and the CRDM unit such that the CRDM unit provides at leastone of gray rod control and shutdown rod control for the at least onecontrol rod; wherein the control rod/CRDM coupling has an averagedensity greater than the density of stainless steel at room temperature.

In another aspect of the disclosure, an apparatus comprises a connectingrod of a control rod assembly of a nuclear reactor. The connecting rodincludes a hollow or partially hollow connecting rod tube comprising afirst material having a first density at room temperature, and a fillerdisposed in the hollow or partially hollow connecting rod tube, thefiller comprising a second material having a second density at roomtemperature that is greater than the first density.

In another aspect of the disclosure, an apparatus comprises a nuclearreactor pressure vessel, and a control rod assembly including at leastone movable control rod comprising a neutron absorbing material, acontrol rod drive mechanism (CRDM) for controlling movement of the atleast one control rod, and a coupling operatively connecting the atleast one control rod and the CRDM. The coupling includes at least aconnecting rod comprising a hollow or partially hollow connecting rodtube comprising a first material having a first density at roomtemperature, and a filler disposed in the hollow or partially hollowconnecting rod tube, the filler comprising a second material having asecond density at room temperature that is greater than the firstdensity.

In another aspect of the disclosure, an apparatus comprises a nuclearreactor pressure vessel, and a control rod assembly including at leastone movable control rod comprising a neutron absorbing material, acontrol rod drive mechanism (CRDM) for controlling movement of the atleast one control rod, and a coupling operatively connecting the atleast one control rod and the CRDM. The coupling includes a firstportion comprising a first material having a first density at roomtemperature, and a second portion comprising a second material having asecond density at room temperature that is greater than the firstdensity.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various process operations and arrangements ofprocess operations. The drawings are only for purposes of illustratingpreferred embodiments and are not to be construed as limiting theinvention.

FIG. 1 diagrammatically shows a perspective sectional view a lowerportion of an illustrative nuclear reactor pressure vessel including anillustrative control rod assembly (CRA).

FIG. 2 diagrammatically shows a perspective view of the illustrative CRAof FIG. 1.

FIG. 3 diagrammatically shows a perspective view of the control rodguide frame with the CRDM unit removed so as to reveal an upper end of aconnecting rod of the CRA.

FIG. 4 diagrammatically shows a perspective view the control rods andthe connecting rod of the CRA of FIGS. 1-3, with components that wouldocclude the view of these components removed.

FIG. 5 diagrammatically shows a perspective view of the terminalweighting element of the CRA of FIGS. 1-4.

FIG. 6 diagrammatically shows a perspective sectional view of theterminal weighting element of FIG. 5.

FIG. 7 diagrammatically shows a top view of a casing of the terminalweighting element of FIGS. 5 and 6.

FIG. 8 diagrammatically shows a top view of the casing of the terminalweighting element of FIGS. 5-7 located in the control rod guide frame ofthe CRA of FIGS. 1-3.

FIG. 9 diagrammatically shows a perspective sectional view of the J-Lockfemale attachment assembly housed or disposed in the central passage ofthe terminal weighting element of FIGS. 5-7.

FIG. 10 diagrammatically shows a perspective view of the assembly of theconnecting rod, terminal weighting element, and control rods includingan upper portion of the J-Lock coupling.

FIG. 11 diagrammatically shows a perspective sectional view of theassembly of the connecting rod, terminal weighting element, and controlrods including details of the J-Lock coupling in its lockedconfiguration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Disclosed herein is a paradigm shift in control rod/CRDM couplingassemblies. In existing control rod/CRDM coupling assemblies, thecontrol rod is terminated by a lightweight, “spidery” spider having aminimal weight and surface area oriented broadside to the SCRAMdirection. The spider is configured to provide a large “effective” areafor attachment of control rods, but a small “actual” area contributingto hydraulic resistance during SCRAM. Both the spider and the connectingrod are stainless steel components so as to provide benefits such asstrength and robustness, low cost, manufacturability, and compatibilitywith the reactor vessel environment.

Disclosed herein are control rod/CRDM coupling assemblies that includeone or both of the following aspects: (i) replacement of theconventional lightweight spider with a terminal weighting element,and/or (ii) replacement of a substantial portion of the stainless steelof the control rod/CRDM coupling assembly with a denser material such astungsten (optionally in a powdered or granulated form), molybdenum,tantalum, or so forth. The disclosed control rod/CRDM couplingassemblies are substantially heavier than conventional connectingrod/spider assemblies, which advantageously enhances the speed andreliability of gravitationally-induced SCRAM.

In the case of control rod/CRDM coupling assemblies employing thedisclosed terminal weighting element, the increased weight provided bythe terminal weighting element as compared with a conventionallightweight spider enables the terminal weighting element to optionallyhave a larger actual surface area broadside to the SCRAM direction (forexample, in order to provide the additional weight) as compared with theconventional spider.

With reference to FIG. 1, a relevant portion of an illustrative nuclearreactor pressure vessel 10 includes a core former 12 located proximateto a bottom of the pressure vessel 10. The core former 12 includes orcontains a reactive core (not shown) containing or including radioactivematerial such as, by way of illustrative example, enriched uranium oxide(that is, UO₂ processed to have an elevated ²³⁵U/²³⁸U ratio). A controlrod drive mechanism (CRDM) unit 14 is diagrammatically illustrated. Theillustrative CRDM 14 is an internal CRDM that is disposed within thepressure vessel 10; alternatively, an external CRDM may be employed.FIG. 1 shows the single illustrated CRDM unit 14 as an illustrativeexample; however, more generally there are typically multiple CRDM unitseach coupled with a different plurality of control rods (although theseadditional CRDM units are not shown in FIG. 1, the pressure vessel 10 isdrawn showing the space for such additional CRDM units).

Below the CRDM unit 14 is a control rod guide frame 16, which in theperspective view of FIG. 1 blocks from view the control rod/CRDMcoupling assembly (not shown in FIG. 1). Extending below the guide frame16 are a plurality of control rods 18. FIG. 1 shows the control rods 18in their fully inserted position in which the control rods 18 aremaximally inserted into the core former 12. In the fully insertedposition, the terminal weighting element (or, in alternativeembodiments, the spider) is located at a lower location 20 within thecontrol rod guide frame 16 (and, again, hence not visible in FIG. 1). Inthe illustrative embodiment of FIG. 1, the CRDM unit 14 and the controlrod guide frame 16 are spaced apart by a standoff 22 comprising a hollowtube having opposite ends coupled with the CRDM unit 14 and the guideframe 16, respectively, and through which the connecting rod (not shownin FIG. 1) passes.

FIG. 1 shows only a lower portion of the illustrative pressure vessel10. In an operating nuclear reactor, an open upper end 24 of theillustration is connected with one or more upper pressure vesselportions that together with the illustrated lower portion of thepressure vessel 10 form an enclosed pressure volume containing thereactor core (indicated by the illustrated core former 12), the controlrods 18, the guide frame 16, and the internal CRDM unit 14. In analternative embodiment, the CRDM unit is external, located above thereactor pressure vessel. In such embodiments, the external CRDM isconnected with the control rods by a control rod/CRDM coupling assemblyin which the connecting rod extends through a portal in the upperportion of the pressure vessel.

With reference to FIG. 2, the control assembly including the CRDM unit14, the control rod guide frame 16, the intervening standoff 22, and thecontrol rods 18 is illustrated isolated from the reactor pressurevessel. Again, the control rod/CRDM coupling assembly is hidden by thecontrol rod guide frame 16 and the standoff 22 in the view of FIG. 2.

With reference to FIG. 3, the control rod guide frame 16 and thestandoff 22 is again illustrated, but with the CRDM unit removed so asto reveal an upper end of a connecting rod 30 extending upwardly abovethe standoff 22. If the CRDM unit has gray rod functionality, then thisillustrated upper end of the connecting rod 30 engages with the CRDMunit to enable the CRDM unit to raise or lower the connecting rod 30and, hence, the attached control rods 18 (not shown in FIG. 3). If theCRDM unit has shutdown rod functionality, then this illustrated upperend is detachable from the CRDM unit during SCRAM. In each of FIGS. 1-4,a SCRAM direction S is indicated, which is the downward direction ofacceleration of the falling control rods in the event of a SCRAM.

With reference to FIG. 4, the control rods 18 and the connecting rod 30are shown without any of the occluding components (e.g., without theguide frame, standoff, or CRDM unit). In the view of FIG. 4 anillustrative terminal weighting element 32 is visible, which providesconnection of the plurality of control rods 18 with the lower end of theconnecting rod 30. It will be noticed that, unlike a conventionalspider, the terminal weighting element 32 has substantial elongationalong the SCRAM direction S. The illustrated terminal weighting element32 has the advantage of providing enhanced weight which facilitatesrapid SCRAM; however, it is also contemplated to replace the illustratedterminal weighting element 32 with a conventional “spidery” spider.

With reference to FIGS. 5 and 6, a perspective view and a side-sectionalperspective view, respectively, of the terminal weighting element 32 isshown. The terminal weighting element 32 includes a substantially hollowcasing 40 having upper and lower ends that are sealed off by upper andlower casing cover plates 42, 44. Four upper casing cover plates 42 areillustrated in FIG. 5 and two of the upper casing cover plates 42 areshown in the side-sectional persective view of FIG. 6. The tilt of theperspective view of FIG. 5 occludes the lower cover plates from view,but two of the lower cover plates 44 are visible “on-edge” in theside-sectional view of FIG. 6. The illustrative terminal weightingelement 32 includes four lower casing cover plates 44 arrangedanalogously to the four upper casing cover plates 42 illustrated in FIG.5.

Further visualization of the illustrative terminal weighting element 32is provided by FIG. 7, which shows a top view of the hollow casing 40with the cover plates omitted. As seen in FIG. 7, the hollow casing 40is cylindrical having a cylinder axis parallel with the SCRAM directionS and a uniform cross-section transverse to the cylinder axis. Thatcross-section is complex, and defines a central passage 50 and fourcavities 52 spaced radially at 90° intervals around the central passage50. The cross-section of the hollow casing 40 also defines twenty-foursmall passages 54 (that is, small compared with the central passage 50),of which only some of the twenty-four small passages 54 are expresslylabeled in FIG. 7. Comparison of FIG. 7 with FIGS. 5 and 6 show that thepassages 50, 54 each pass completely through the casing 50 and are notcovered by the upper or lower cover plates 42, 44.

Considering first the twenty-four small passages 54, these providestructures for securing the plurality of control rods 18. In someembodiments, each of the twenty-four of the small passages 54 retain acontrol rod, such that the plurality of control rods 18 consists ofprecisely twenty-four control rods. In other embodiments, one or more ofthe twenty-four small passages 54 may be empty or may be used foranother purpose, such as being used as a conduit for in-coreinstrumentation wiring, in which case the plurality of control rods 18consists of fewer than twenty-four control rods. It is to be furtherappreciated that the terminal weighting element 32 is merely anillustrative example, and that the terminal weighting element may haveother cross-sectional configurations that provide for different numbersof control rods, e.g. more or fewer than twenty-four.

The four cavities 52 spaced radially at 90° intervals around the centralpassage 50 are next considered. The substantially hollow casing 40 andthe upper and lower cover plates 42, 44 are suitably made of stainlesssteel, although other materials are also contemplated. The upper andlower cover plates 42, 44 seal the four cavities 52. As shown in theside-sectional view of FIG. 6, the four cavities 52 are filled with afiller 56 comprising a heavy material, where the term “heavy material”denotes a material that has a higher density than the stainless steel(or other material) that forms the hollow casing 40. For example, thefiller 56 may comprise a heavy material such as tungsten (optionally ina powdered or granulated form), depleted uranium, molybdenum, ortantalum, by way of some illustrative examples. By way of illustrativeexample, stainless steel has a density of about 7.5-8.1 grams/cubiccentimeter, while tungsten has a density of about 19.2 grams/cubiccentimeter and tantalum has a density of about 16.6 grams per cubiccentimeter. In some preferred embodiments, the heavy material comprisingthe filler 56 has a density that is at least twice the density of thematerial comprising the casing 40. In some preferred embodiments inwhich the casing 40 comprises stainless steel, the heavy materialcomprising the filler 56 preferably has a density that is at least 16.2grams per cubic centimeter. (All quantitative densities specified hereinare for room temperature.)

In some embodiments, the filler 56 does not contribute to the structuralstrength or rigidity of the terminal weighting element 32. Accordingly,heavy material comprising the filler 56 can be selected withoutconsideration of its mechanical properties. For the same reason, thefiller 56 can be in the form of solid inserts sized and shaped to fitinto the cavities 52, or the filler 56 can be a powder, granulation, orother constitution. The cover plates 42, 44 seal the cavities 52, and soit is also contemplated for the heavy material comprising the filler 56to be a material that is not compatible with the primary coolant flowingin the pressure vessel 10. Alternatively, if the heavy materialcomprising the filler 56 is a material that is compatible with theprimary coolant flowing in the pressure vessel 10, then it iscontemplated to omit the upper cover plates 42, in which case thecavities 52 are not sealed. Indeed, if the filler 56 is a solid materialsecurely held inside the cavities 52, then it is contemplated to omitboth the upper cover plates 42 and the lower cover plates 44.

With continuing reference to FIGS. 5-7 and with further reference toFIG. 8, the terminal weighting element 32 passes through the control rodguide frame 16 as the control rods 18 are raised or lowered by action ofthe CRDM unit 14. The cylindrical configuration with constantcross-section over the length of the terminal weighting element 32 alongthe SCRAM direction S simplifies this design aspect. Moreover, thecontrol rod guide frame 16 should cam against each control rod 18 toprovide the desired control rod guidance. Toward this end, thecross-section of the terminal weighting element 32 is designed withrecesses 58 (some of which are labeled in FIG. 7). As shown in FIG. 8,into these recesses 58 fit mating extensions 60 of the control rod guideframe 16. A gap G also indicated in FIG. 8 provides a small tolerancebetween the outer surface of the terminal weighting element 32 and theproximate surface of the control rod guide frame 16. The twenty-fourpartial circular openings of the guide frame 16 which encompass thetwenty-four small passages 54 of the terminal weighting element 32 aresized to cam against the control rods 18. For completeness, FIG. 8 alsoshows the connecting rod 30 disposed inside the central passage 50 ofthe terminal weighting element 32.

FIGS. 5-7 show that providing space for the four cavities 52substantially increases the actual cross-sectional area of the terminalweighting element 32 (that is, the area arranged broadside to the SCRAMdirection S), as compared with the actual cross-sectional area thatcould be achieved without these four cavities 52. In some embodiments,the “fill factor” for the cross-section oriented broadside to the SCRAMdirection S (including the area encompassed by the cover plates 42, 44)is at least 50%, and FIG. 7 demonstrates that the fill factor issubstantially greater than 50% for the illustrative terminal weightingelement. Thus, the design of the terminal weighting element 32 isdistinct from the “spidery” design of a typical spider, which isoptimized to minimize the actual surface area broadside to the SCRAMdirection S and generally has a fill factor of substantially less than50% in order to reduce hydraulic resistance. In general, the SCRAM forceachieved by the weight of the terminal weighting element 32 more thanoffsets the increased hydraulic resistance of the greater actualbroadside surface area imposed by the four cavities 52.

Additional weight to overcome the hydraulic resistance and enhance SCRAMspeed is obtained by elongating the terminal weighting element 32 in theSCRAM direction S. Said another way, a ratio of a length of the terminalweighting element 32 in the SCRAM direction S versus the largestdimension oriented broadside to the SCRAM direction S is optionallyequal to or greater than one, and is more preferably equal to or greaterthan 1.2. The illustrative terminal weighting element 32 is not agenerally planar element as per a typical spider, but rather is avolumetric component that provides substantial terminal weight to thelower end of the connecting rod 30.

The illustrative terminal weighting element 32 has a substantialadvantage in that it places the filler 56 comprising heavy materialbetween the radioactive core (contained in or supported by the coreformer 12 located proximate to the bottom of the pressure vessel 10 asshown in FIG. 1) and the CRDM unit 14. The heavy material comprising thefiller 56 is a dense material which can generally be expected to behighly absorbing for radiation generated by the reactor core. Highradiation absorption is a property of heavy materials such as tungsten,depleted uranium, molybdenum, or tantalum, by way of illustrativeexample. Thus, the filler 56 comprising heavy material providesradiation shielding that protects the expensive and (in some embodimentsand to various extent) radiation-sensitive CRDM unit 14.

The elongation of the terminal weighting element 32 in the SCRAMdirection S has additional benefits that are independent of providingweight. The elongation in the SCRAM direction S provides a longer lengthover which each control rod 18 can be secured to the terminal weightingelement 32, and similarly provides a longer length over which theconnecting rod 30 can be secured to the terminal weighting element 32.This provides a better mechanical couplings, and also provides enhancedstabilizing torque to prevent the control rods 18 from tilting. Ingeneral, the elongation of the terminal weighting element 32 in theSCRAM direction S provides a more rigid mechanical structure thatreduces the likelihood of problematic (or even catastrophic) deformationof the connecting rod/terminal weighting element/control rods assembly.

Another advantage of the elongation of the terminal weighting element 32in the SCRAM direction S is that it optionally allows for streamliningthe terminal weighting element 32 in the SCRAM direction S. Thisvariation is not illustrated; however, it is contemplated to modify theconfiguration of FIG. 5 (by way of illustrative example) to have anarrower lower cross-section and a broader upper cross section, with aconical surface of increasing diameter running from the narrower lowercross-section to the broader upper cross section. The small passages 54for securing the control rods would remain oriented precisely parallelwith the SCRAM direction S (and, hence, would be shorter for controlrods located at the outermost positions). Such streamlining represents atrade-off between hydraulic resistance (reduced by the streamlining) andweight reduction caused by the streamlining.

Instead of the mentioned optional streamlining, the cross-section of theterminal weighting element can be otherwise configured to reducehydraulic resistance. For example, the cross-section can includeadditional passages (not shown) analogous to the small passages 54, butwhich are not filled with control rods or anything else, and insteadprovide fluid flow paths to reduce the hydraulic resistance of theterminal weighting element during a SCRAM.

The illustrative terminal weighting element 32 provides a desired weightby a combination of the filler 56 comprising a heavy material (whichincreases the average density of the terminal weighting element 32 to avalue greater than the average density of stainless steel) and theelongation of the terminal weighting element 32 (which increases thetotal volume of the terminal weighting element 32). The total mass(equivalent to weight) is given by the product of the volume and theaverage density. To achieve a desired weight, various design trade-offscan be made amongst: (1) the size or amount or volume of the filler 56;(2) the density of the heavy material comprising the filler 56; and (3)the elongation of the terminal weighting element 32.

In some embodiments, it is contemplated to achieve the desired weight byusing a filler comprising a heavy material without elongating theterminal weighting element. In such embodiments, the terminal weightingelement 32 may optionally have a conventional substantially planar and“spidery” spider configuration, in which the tubes or other connectingelements of the spider are partially or wholly hollow to define cavitiescontaining the filler comprising a heavy material. Such a terminalweighting element can be thought of as a “heavy spider”.

In other embodiments, it is contemplated to omit the filler materialentirely, and instead to rely entirely upon elongation to provide thedesired weight. For example, the illustrated terminal weighting element32 can be modified by omitting the four cavities 52 and the filler 56.In this configuration the casing 40 can be replaced by a single solidstainless steel element having the same outer perimeter as the casing40, with the top and bottom of the single solid stainless steel elementdefining (or perhaps better stated, replacing) the upper and lowercasing cover plates 42, 44. Such embodiments omitting the fillercomprising heavy material are suitably employed if the elongatedterminal weighting element 32 made entirely of stainless steel providessufficient weight. Such embodiments are also suitably employed if theweight of the terminal element is not a consideration, but otherbenefits of the elongated terminal element are desired, such asproviding a longer length for secure connection with the control rodsand/or the connecting rod 30, or providing an elongated geometry in theSCRAM direction S which is amenable to streamlining.

Various embodiments of the disclosed terminal weighting elements use astainless steel casing that does not compromise the primary function ofproviding a suitable structure for coupling the control rods to thelower end of the connecting rod. At the same time, the stainless steelcasing leaves sufficient void or cavity volume to allow a fillercomprising a heavy material to be inserted. Although stainless steel isreferenced as a preferred material for the casing, it is to beunderstood that other materials having desired structuralcharacteristics and reactor pressure vessel compatibility can also beused. The filler comprising heavy material is suitably tungsten,depleted uranium, or another suitably dense material.

Various embodiments of the disclosed terminal weighting elements alsohave elongation in the SCRAM direction S. This elongated design isreadily configured to fit into the control rod guide frame without anyredesign (e.g., widening) of the guide frame, and hence does not impactthe space envelope of the overall control rod assembly. The elongationis an adjustable design parameter, and can be set larger or smaller toprovide the desired weight. Increasing the elongation generallyincreases the control rod assembly height, and this may impose an upperlimit on the elongation for a particular reactor design. (This may be atleast partially compensated by reducing the connecting rod length, butthe connecting rod has a minimum length imposed by the desired maximumtravel).

Another advantage of the disclosed terminal weighting element is that itcan provide adjustable weight. For example, in some embodimentsdifferent CRDM units may be located at different heights, or may supportcontrol rods of different masses, such that the different translatingassemblies associated with the different CRDM units are not identical.If it is deemed beneficial for all translating assemblies associatedwith the various CRDM units to have the same weight, then differentamounts of the filler comprising heavy material can be included in thecavities 52 of different terminal weighting elements 32 in order toequalize the weights of the translating assemblies. In some cases thismight result in some cavities 52 being only partially filled with thefiller 56. Optionally, the unfilled space of the cavities 52 can befilled with a light weight filler material such as a stainless steelslug (not shown) or can contain a compressed loading spring (not shown)to prevent the filler 56 comprising heavy material from moving aboutwithin the cavities 52. The weight of the light weight filler or loadingspring is suitably taken into account in selecting the amount of filler56 of heavy material to achieve a desired overall weight. Equalizingweights of the various translating assemblies can be useful, by way ofexample, to allow the use of a common plunger or other kinetic energyabsorbing element in each translating assembly. The kinetic energyabsorbing element (not shown in FIGS. 5-8) is designed to provide a“soft stop” to a translating assembly undergoing SCRAM when the controlrods reach the point of full (i.e., maximal) insertion.

The casing 40 of the illustrative terminal weighting element 32 acts asthe structural part providing mechanical support. All loads associatedwith the coupling between the connecting rod 30 and the control rods 18are transferred into the casing 40 which serves as the attachmentlocation for each control rod.

With reference to FIGS. 9, 10, and 11, various attachment configurationscan be used for securing the connecting rod 30 in the attachment passage50 of the casing 40 of the terminal weighting element 32. In anillustrative example of one such attachment configuration, the centralpassage 50 of the casing 40 houses a J-Lock female attachment assembly70, which is suitably coaxially disposed inside the central passage 50of the casing 40. FIG. 9 illustrates a side sectional view of the J-Lockfemale attachment assembly 70, while FIG. 10 shows a side view of theconnected assembly and FIG. 11 shows a side sectional view of theconnected assembly. With particular reference to FIG. 9, theillustrative J-Lock female attachment assembly 70 includes a hub 72which in the illustrative embodiment comprises a round cylindercoaxially welded or otherwise secured in the central passage 50 of thecasing 40. Alternatively, the hub may be integral with or defined by aninside surface of the central passage 50. The hub 72 serves as aninterface between the casing 40 and the J-Lock female attachmentcomponents, which include three J-Lock pins 74 (two of which visible inthe sectional view of FIG. 9) disposed inside of the hub 72. These pins74 provide the connection points for a J-Lock male attachment assembly80 (see FIG. 11) disposed at the lower end of the connecting rod 30. AJ-Lock plunger 76 and a J-Lock spring 84 keeps the J-Lock maleattachment assembly 80 of the connecting rod 30 in place once it hasbeen engaged with the terminal weighting element 32. (Locked arrangementshown in FIG. 11).

The illustrative J-Lock female attachment assembly 70 further includes alower plunger 82, an inner spring 78, and a spring washer 86 whichcooperate to absorb the impact of the lower translating assembly (thatis, the translating combination of the control rods 18, the terminalweighting element 32, the connecting rod 30, and optionally a lead screw(not shown)) during a SCRAM.

The illustrative J-Lock connection between the lower end of theconnecting rod 30 and the terminal weighting element 32 is an example.More generally, substantially any type of connection, including anothertype of detachable connection or a permanently welded connection or anintegral arrangement, is contemplated. The J-Lock arrangement has theadvantage of enabling the connecting rod 30 to be detached from theterminal weighting element 32 (and, hence, from the control rods 18) bya simple “push-and-twist” operation. This allows the connecting rod 30to be moved separately from the remainder of the translating assembly(that is, the terminal weighting element 32 and the attached controlrods 18) during refueling of the nuclear reactor.

The casing 40 of the terminal weighting element 32 can be manufacturedusing various techniques. In some embodiments manufacturing employingElectrical Discharge Machining (EDM) is contemplated. The EDM methodoperates on a solid block of stainless steel which is then cut to definethe spider casing 40. Advantageously, EDM is fast and precise. Othercontemplated methods include casting techniques or extrusion, both ofwhich are fast and have low material cost.

The translating assembly comprising the control rods 18, terminalweighting element 32, connecting rod 30, and optionally a lead screw(not illustrated) is advantageously heavy in order to facilitate rapidand reliable SCRAM of the translating assembly toward the reactor corein the event of an emergency reactor shutdown. Toward this end, theterminal weighting element 32 is configured to be heavy. One waydisclosed herein to achieve this is by increasing the average density ofthe terminal weighting element 32 to a value greater than that ofstainless steel (or, more generally, increasing its average density to avalue greater than that of the material comprising the casing 40) by theaddition of the filler 56 comprising heavy material (where “heavy”denotes a density greater than that of the stainless steel or othermaterial comprising the casing 40). Another way disclosed herein toachieve this is by elongating the terminal weighting element 32 in theSCRAM direction S. The illustrative terminal weighting element 32employs both enhanced average density via filler 56 and elongation inthe SCRAM direction S.

With reference to FIGS. 10 and 11, additional weight for the translatingassembly is additionally or alternatively obtained by enhancing thedensity of the connecting rod 30. Toward this end, the illustrativeconnecting rod 30 includes a hollow (or partially hollow) connecting rodtube 90 which (as seen in the sectional view of FIG. 11) contains afiller 92 comprising heavy material. Thus, the connecting rod tube 90serves the structural purpose analogous to the casing 40 of the terminalweighting element 32, while the filler 92 comprising heavy materialserves a weighting (or average density-enhancing) purpose analogous tothe filler 56 of the terminal weighting element 32. The hollowconnecting rod tube 90 can be manufactured using various techniques,such as EDM (although longer tube lengths may be problematic for thisapproach), casting, extrusion, milling, or so forth.

In one suitable embodiment, the filler 92 comprising heavy material isin the form of tungsten slugs each having a diameter substantiallycoinciding with an inner diameter of the connecting rod tube 90 andbeing stacked in the connecting rod tube 90, with the number of stackedtungsten slugs being selected to achieve the desired weight. If thenumber of tungsten slugs is insufficient to fill the interior volume ofthe connecting rod tube 90 and it is desired to avoid movement of theseslugs, then optionally the filler 92 is prevented from shifting by asuitable biasing arrangement or by filling the remaining space withinthe interior volume of the connecting rod tube 90 with a light weightmaterial such as stainless steel slugs. In the illustrative example ofFIG. 11, a biasing arrangement is employed, in which the interior volumeof the connecting rod tube 90 is sealed off by upper and lower weldedplugs 94, 96, and a compressed spring 98 takes up any slack along theSCRAM direction S that may be introduced by incomplete filling of theinterior volume of the connecting rod tube 90 by the filler 92. Insteadof tungsten, the heavy material comprising the filler may be depleteduranium, molybdenum, tantalum, or so forth, by way of some otherillustrative examples. The filler 92 may comprise one or more solidslugs or rods, a powder, a granulation, or so forth. In the context ofthe connecting rod 30, the term “heavy material” refers to a materialhaving a density that is greater than the density of the stainless steelor other material comprising the connecting rod tube 90. By way ofillustrative example, stainless steel has a density of about 7.5-8.1grams/cubic centimeter, while tungsten has a density of about 19.2grams/cubic centimeter and tantalum has a density of about 16.6 gramsper cubic centimeter. In some preferred embodiments, the heavy materialcomprising the filler 92 has a density that is at least twice thedensity of the material comprising the hollow connecting rod tube 90. Insome preferred embodiments in which the hollow connecting rod tube 90comprises stainless steel, the heavy material comprising the filler 92preferably has a density that is at least 16.2 grams per cubiccentimeter. (All quantitative densities specified herein are for roomtemperature.)

With continuing reference to FIGS. 10 and 11, the illustrativeconnecting rod 30 has an upper end that includes an annular groove 100for securing with a latch of the CRDM unit 14 (latch not shown), and amagnet 102 for use in conjunction with a control rod position sensor(not shown). A suitable embodiment of the CRDM unit 14 including amotor/lead screw arrangement for continuous (gray rod) adjustment and aseparate latch for detaching the connecting rod 30 from the CRDM unit 14(with the lead screw remaining operatively connected with the motor) isdescribed in related application Ser. No. 12/722,662 titled “Control RodDrive Mechanism For Nuclear Reactor” filed Mar. 12, 2010 and relatedapplication Ser. No. 12/722,696 titled “Control Rod Drive Mechanism ForNuclear Reactor” filed Mar. 12, 2010, both of which are bothincorporated herein by reference in their entireties.

Alternatively, in other embodiments a lead screw (not shown) is securedwith or integral with the connecting rod tube 90, and the lead screwSCRAMs together with the connecting rod/terminal weighting element (orspider)/control rod (in other words, the lead screw forms part of thetranslating assembly during SCRAM). In some such alternativeembodiments, the motor is suitably coupled with the lead screw by aseparable ball nut that separates to release the lead screw and initiateSCRAM.

The illustrative connecting rod 30 includes eight components. The weightof the connecting rod 30 assembly is increased by using the hollowconnecting rod tube 90. This may be only partially hollow—for example,only a lower portion may be hollow. Located inside the hollow connectingrod tube 90 is the filler 92 comprising heavy material. In someembodiments, the filler 92 comprises several smaller rods or slugs oftungsten. The number of tungsten rods or slugs inside the hollowconnecting rod tube 90 is selected to achieve a desired weight. Ifdifferent translating assemblies are employed with different CDRM units,the number of tungsten rods or slugs inside each of the hollowconnecting rod tubes 90 may be different, and selected so as to ensurethat each connecting rod of the several CDRM units has the same weight.This is advantagous since it follows that all of the CRDM units can bedesigned to lift a single weight independent of factors such asconnecting rod length, control rod composition, or so forth.

As already noted, such weight “tuning” can also be achieved by adjustingthe filler 56 in the terminal weighting element 32. If both fillers 56,92 are employed, then the combined weight of the fillers 56, 92 can betuned by adjusting the amount and/or density of either one, or both, ofthe fillers 56, 92. If the amount of weight tuning is expected to besmall, then in some such embodiments the fillers 56, 92 may be solidelements of standard size/weight, and the total weight may then betrimmed by adding additional filler comprising heavy material in theform of a powder, granulation, small slug or slugs, or so forth.

If the interior volume of the hollow connecting rod tube 90 is onlypartially filled by the filler 92, then stainless steel rods or someother light weight filler (not shown) may be inserted into the remaininginterior volume to fill complete the filling. Additionally oralternatively, the spring 98 or another mechanical biasing arrangementmay be employed. It is contemplated to have the filler 92 arranged“loosely” in the rod tube 90; however, such an arrangement maycomplicate absorption of kinetic energy at the termination of a SCRAMdrop.

The filler 92 generally has a lower coefficient of thermal expansionthan the stainless steel (or other material) of the hollow connectingrod tube 90. The connecting rod 30 is assembled at room temperature, andthen heated to its operating temperature. For a connecting rod having alength of, e.g. 250 centimeters or greater, the thermal expansion willresult in the rod tube 90 increasing by an amount of order a fewcentimeters or more. The lower coefficient of thermal expansion of thefiller 92 results in a substantially lower length increase of the filler92. The spring 98 suitably compensates for this effect. Additionally, ifthe spring 98 is located below the filler 92 (as shown in FIG. 11), thenit can assist in dissipating the kinetic energy of the filler 92 at thetermination of the SCRAM drop.

As shown in the illustrative embodiment depicted in FIG. 11, the hollowconnecting rod tube 90 may be less than the total length of theconnecting rod 30. In the illustrated case, the connecting rod 30includes additional length below the rod tube 90 corresponding to theJ-Lock male attachment assembly 80, and also includes additional lengthabove the rod tube 90 corresponding to an upper tube that includes thelatch groove 100 and houses the position indicator magnet 102. The upperand lower welded plugs 94, 96 are optionally provided to seal off theinterior volume of the hollow connecting rod tube 90. These plugs 94, 96are attached to the upper and lower ends, respectively of the hollowconnecting rod tube 90 so as to seal the filler 92 and the optionalspring 98 inside. In the illustrative embodiment, the outer ends of theplugs 94, 96 are configured to facilitate connection of the upperconnecting rod and the J-lock male attachment assembly 80, respectively.

The connecting rod 30 also has a substantial advantage in that it placesthe filler 92 comprising heavy material between the radioactive core(contained in or supported by the core former 12 located proximate tothe bottom of the pressure vessel 10 as shown in FIG. 1) and the CRDMunit 14. The heavy material comprising the filler 92 is a dense materialwhich can generally be expected to be highly absorbing for radiationgenerated by the reactor core. High radiation absorption is a propertyof heavy materials such as tungsten, depleted uranium, molybdenum, ortantalum, by way of illustrative example. Thus, the filler 92 comprisingheavy material provides radiation shielding that protects the expensiveand (in some embodiments and to various extent) radiation-sensitive CRDMunit 14. If both fillers 56, 92 are used, then both fillers contributeto this advantageous CRDM shielding effect.

The illustrative control rod/CRDM coupling includes a combination of (1)the terminal weighting element 32 including elongation and the filler56, and (2) the connecting rod 30 including the filler 92.

In other control rod/CRDM coupling embodiments it is contemplated toinclude a combination of the terminal weighting element 32 includingelongation and the filler 56 but coupled with a conventional solidstainless steel connecting rod (without the filler 92).

In other control rod/CRDM coupling embodiments it is contemplated toinclude a combination of a terminal element (which may or may not be aweighting element) including elongation but without the filler 56,coupled either with (i) the connecting rod 30 including the filler 92 or(ii) a conventional solid stainless steel connecting rod (without thefiller 92).

In other control rod/CRDM coupling embodiments it is contemplated toinclude a combination of a terminal weighting element without elongation(for example, having a “spidery” topology similar to a conventionalspider) but which includes the filler 56 disposed in hollow regions ofthe tubes or other members of the terminal weighting element, coupledeither with (i) the connecting rod 30 including the filler 92 or (ii) aconventional solid stainless steel connecting rod (without the filler92).

In other control rod/CRDM coupling embodiments it is contemplated toinclude a combination of (I) a conventional spider without elongationand without the filler 56 and (II) the connecting rod 30 including thefiller 92.

The preferred embodiments have been illustrated and described.Obviously, modifications and alterations will occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

We claim:
 1. An apparatus comprising: at least one control rodcomprising a neutron absorbing material; a control rod drive mechanism(CRDM) unit; and a control rod/CRDM coupling connecting the control rodand the CRDM unit such that the CRDM unit provides at least one of grayrod control and shutdown rod control for the at least one control rod;wherein the control rod/CRDM coupling includes a connecting rod havingan average density greater than the density of stainless steel at roomtemperature, wherein the connecting rod comprises: a hollow or partiallyhollow connecting rod tube; a filler disposed in the interior volume ofthe hollow or partially hollow connecting rod tube, the fillercomprising a material having a density greater than the density ofstainless steel at room temperature, and a welded plug sealing off theinterior volume of the hollow or partially hollow connecting rod tubewith the filler disposed in the interior volume.
 2. The apparatus as setforth in claim 1, wherein the hollow or partially hollow connecting rodtube is made of stainless steel.
 3. The apparatus as set forth in claim1, wherein the filler has a density of at least 16.2 grams per cubiccentimeter at room temperature.
 4. The apparatus as set forth in claim1, wherein the hollow or partially hollow connecting rod tube comprisesa material having a first density and the material comprising the fillerhas a density that is at least twice the first density.
 5. The apparatusas set forth in claim 1, wherein the connecting rod further comprises:an additional element disposed in the hollow or partially hollowconnecting rod tube, the additional element not being the fillercomprising a material having a density greater than the density ofstainless steel at room temperature, the additional element preventingthe filler from moving inside the hollow or partially hollow connectingrod tube.
 6. The apparatus as set forth in claim 1, wherein theconnecting rod further comprises: a compressed spring disposed in thehollow or partially hollow connecting rod tube and pressing against thefiller.
 7. The apparatus as set forth in claim 6, wherein the spring isdisposed below the filler in the hollow or partially hollow connectingrod tube such that the spring at least contributes to dissipatingkinetic energy accrued by the filler during a SCRAM.
 8. The apparatus asset forth in claim 3, wherein the hollow or partially hollow connectingrod tube is made of stainless steel.
 9. The apparatus as set forth inclaim 2, wherein the filler comprises a material selected from the groupconsisting of tungsten, depleted uranium, molybdenum, and tantalum. 10.The apparatus as set forth in claim 1, wherein the filler comprises amaterial selected from the group consisting of tungsten, depleteduranium, molybdenum, and tantalum.
 11. An apparatus comprising: at leastone control rod comprising a neutron absorbing material; a control roddrive mechanism (CRDM) unit; and a connecting rod having its lower endconnected with the control rod and its upper end connected with the CRDMunit such that the CRDM unit provides at least one of gray rod controland shutdown rod control for the at least one control rod; wherein theconnecting rod comprises (i) a hollow or partially hollow stainlesssteel rod tube having a length of 250 centimeters or greater and (ii) afiller material disposed inside the hollow or partially hollow stainlesssteel rod tube, the filler material having a density greater than thedensity of stainless steel at room temperature.
 12. The apparatus ofclaim 11 wherein the at least one control rod comprises a plurality ofcontrol rods and the apparatus further comprises: a spider via which thelower end of the connecting rod is connected with the plurality ofcontrol rods.
 13. An apparatus comprising: at least one control rodcomprising a neutron absorbing material; a control rod drive mechanism(CRDM) unit; a connecting rod having its lower end connected with thecontrol rod and its upper end connected with the CRDM unit such that theCRDM unit provides at least one of gray rod control and shutdown rodcontrol for the at least one control rod; and a detachable attachmentassembly via which the lower end of the connecting rod connects with theat least one control rod, the detachable attachment assembly includingmating male and female attachment components; wherein the connecting rodcomprises (i) a hollow or partially hollow rod tube made of a rod tubematerial and (ii) a heavy material disposed inside the hollow orpartially hollow rod tube, the heavy material having a density greaterthan the density of the rod tube material, the heavy material disposedinside the hollow or partially hollow rod tube being different from themale and female attachment components of the detachable attachmentassembly.
 14. The apparatus of claim 13 wherein the rod tube material isstainless steel.
 15. The apparatus of claim 14 wherein the heavymaterial comprises tungsten.
 16. The apparatus of claim 14 wherein theheavy material comprises depleted uranium.
 17. The apparatus of claim 14wherein the heavy material comprises molybdenum.
 18. The apparatus ofclaim 14 wherein the heavy material comprises tantalum.
 19. Theapparatus of claim 13 wherein the heavy material comprises tungsten. 20.The apparatus of claim 13 wherein the heavy material comprises depleteduranium.
 21. The apparatus of claim 13 wherein the heavy materialcomprises molybdenum.
 22. The apparatus of claim 13 wherein the heavymaterial comprises tantalum.
 23. The apparatus of claim 13 wherein theheavy material has a density that is at least twice the density of therod tube material.
 24. The apparatus of claim 23 wherein the rod tubematerial is stainless steel.
 25. The apparatus of claim 13 wherein theat least one control rod comprises a plurality of control rods and theapparatus further comprises: a spider via which the lower end of theconnecting rod is connected with the plurality of control rods, thespider including the female attachment component of the detachableattachment assembly and the lower end of the connecting rod includingthe male attachment component of the detachable attachment assembly. 26.The apparatus as set forth in claim 1, wherein the hollow or partiallyhollow connecting rod tube has a length of 250 centimeters or greater.27. The apparatus as set forth in claim 13, wherein the hollow orpartially hollow connecting rod tube has a length of 250 centimeters orgreater.
 28. The apparatus as set forth in claim 11, wherein theconnecting rod further comprises: a welded plug sealing off the interiorvolume of the hollow or partially hollow connecting rod tube.
 29. Theapparatus as set forth in claim 13, wherein the connecting rod furthercomprises: a welded plug sealing off the interior volume of the hollowor partially hollow connecting rod tube.
 30. The apparatus as set forthin claim 1, further comprising: a detachable attachment assembly viawhich a lower end of the connecting rod connects with the at least onecontrol rod, the detachable attachment assembly including mating maleand female attachment components; wherein the filler disposed in thehollow or partially hollow rod tube is different from the male andfemale attachment components of the detachable attachment assembly. 31.The apparatus as set forth in claim 11, further comprising: a detachableattachment assembly via which the lower end of the connecting rodconnects with the at least one control rod, the detachable attachmentassembly including mating male and female attachment components; whereinthe filler disposed inside the hollow or partially hollow stainlesssteel rod tube is different from the male and female attachmentcomponents of the detachable attachment assembly.
 32. An apparatuscomprising: at least one control rod comprising a neutron absorbingmaterial; a control rod drive mechanism (CRDM) unit; and a connectingrod having its lower end connected with the control rod and its upperend connected with the CRDM unit such that the CRDM unit provides atleast one of gray rod control and shutdown rod control for the at leastone control rod; wherein the connecting rod comprises (i) a hollow orpartially hollow rod tube made of a rod tube material and (ii) a heavymaterial comprising depleted uranium disposed inside the hollow orpartially hollow rod tube, the heavy material having a density greaterthan the density of the rod tube material.
 33. The apparatus of claim 32wherein the rod tube material is stainless steel.